The D4 dopamine receptor, one of at least five subtypes of dopamine receptors, was recently cloned. Mounting evidence suggests that many variants of this receptor are found in human populations. One variant of the receptor occurs with high frequency in a population with attention deficit hyperactivity disorder. The D4 receptor is also relevant to schizophrenia, as the effective antipsychotic drug clozapine binds the receptor with high affinity. In cocaine addiction, the D4 receptor is implicated as an indirect target of cocaine. For each of these neuropsychiatric disorders, investigation of D4 dopamine receptor pharmacology, receptor expression and regulation is warranted. Although it is feasible to study the D4 receptor in cell lines expressing the cloned receptor. the functional significance, distribution, binding properties, expression and regulation of the D4 receptor in primate brain tissue has been difficult because of inadequate probes for this receptor subtype. The purpose of the present study was to investigate the brain distribution and pharmacological properties of the D4 receptor in primate brain, using two novel probes for the D4 dopamine receptor. Autoradiographic distribution of the receptor was determined in rhesus monkey brain (Macaca mulatta) with a D4-selective probe [3H]LU-85-295. Initial parametric studies were conducted to establish appropriate incubation and buffer wash times. Subsequently, coronal tissue sections were incubated with [3H]LU 85295 (3 nM) to measure total binding and with haloperidol (10 fM) to measure nonspecific binding. Autoradiography with [3H]LU 85295 revealed a brain distribution of the D4 receptor similar to the distribution of mRNA encoding the receptor, with highest levels in frontal and insular cortices and in hippocampus, amygdala, thalamus, and hypothalamus. Stability problems with [3H]LU 85295 prompted us to custom synthesize a novel probe [3H]U-101958 for the D4 receptor. Preliminary data in homogenates of hippocampus indicate that the binding properties of [3H]-LU 85295 are consistent with the D4 receptor. This research is the first to identify the D4 receptor in primate brain tissue with a D4-selective probe. Research to fully characterize and quantify the receptor in primate brain are ongoing. These studies may provide effective leads for investigating regulation of the receptor in living and in post-mortem human brain. DeLa Garza II, R and Madras BK. Autoradiographic distribution of dopamine D4 receptors in non-human primate brain. Soc. Neurosci.